ruby objects, classes and variables cs 480/680 – comparative languages
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Ruby Objects, Classes and VariablesRuby Objects, Classes and Variables
CS 480/680 – Comparative LanguagesCS 480/680 – Comparative Languages
Ruby Classes 2
Ruby ClassesRuby Classes Remember, in Ruby variables are created
automatically the first time they are accessed Thus, there is no variable declaration section in
a Ruby class, variables are created in the initialize method (the equivalent of a C++ constructor)class Song def initialize(name, artist, duration) @name = name @artist = artist @duration = duration endend
Ruby Classes 3
Using the ClassUsing the Class
aSong = Song.new("Bicylops", "Fleck", 260)aSong.inspect
»"#<Song:0x401b299c @artist=\"Fleck\",@name=\"Bicylops\", @duration=260>"
aSong.to_s»"#<Song:0x401b2a14>"
How can we call to_s and inspect, when we have not defined them for this class?
Answer: All classes are subclasses of class Object• to_s and inspect are inherited from Object
Ruby Classes 4
Overriding the SuperclassOverriding the Superclass
Classes can be reopened any time• Which means that you can override or extend built-
in classes just by opening them
Notice the missing return() in to_s
class Song def to_s "Song: #{@name}--#{@artist} (#{@duration})“ endEnd
aSong = Song.new("Bicylops", "Fleck", 260)aSong.to_s
»"Song: Bicylops--Fleck (260)"
Ruby Classes 5
InheritanceInheritance
“< Song” – indicates that KaraokeSong is a subclass of Song. All methods of Song are included in KaraokeSong• Data members are not explicitly included, but the
are created by the call to super in initialize
class KaraokeSong < Song def initialize(name, artist, duration, lyrics) super(name, artist, duration) @lyrics = lyrics endend
Ruby Classes 6
Calling the SuperclassCalling the Superclass
Calling super with no arguments calls the same-named method in the superclass with the same arguments
class KaraokeSong < Song # Format ourselves as a string by appending # our lyrics to our parent's #to_s value. def to_s super + " [#{@lyrics}]" end end aSong = KaraokeSong.new("My Way", "Sinatra", 225, "And now...") aSong.to_s
» "Song: My Way--Sinatra (225) [And now...]"
Ruby Classes 7
Accessing Data MembersAccessing Data Membersclass Song attr_reader :name, :artist, :duration end
Is the same as this:class Song def name @name end def artist @artist end def duration @duration end end
Ruby Classes 8
Writing Data MembersWriting Data Membersclass Song attr_writer :durationend
Is the same as this:class Song def duration=(newDuration) # Methods ending @duration = newDuration # in “=“ are end # specialend
aSong = Song.new("Bicylops", "Fleck", 260) aSong.duration » 260 aSong.duration = 257aSong.duration » 257
Ruby Classes 9
Class state is protectedClass state is protected In Ruby, you can only access data members
(“state”) of a class through class methodsclass Myclass def initialize() @state1 = 0 @state2 = 1 end
attr_reader(:state1, :state2)end
myobj = Myclass.newputs myobj.state1, myobj.state2
myobj.state1 = 7
objectstate.rb:13: undefined method `state1=' for #<Myclass:0x402b0ed0 @state2=1, @state1=0> (NoMethodError)
Ruby Classes 10
Virtual Data MembersVirtual Data Members When using access methods, data can be
converted before reporting, so data members can be accessed in multiple ways:class Song def durationInMinutes @duration/60.0 # force floating point end def durationInMinutes=(value) @duration = (value*60).to_i end end aSong = Song.new("Bicylops", "Fleck", 260) aSong.durationInMinutes » 4.333333333 aSong.durationInMinutes = 4.2 aSong.duration » 252
Ruby Classes 11
Class VariablesClass Variables Class variables store data stored among all
instances of a class• There is only one class variable storage location for
the entire class• Must be initialized before use in the class definition
Class variables start with @@
Ruby Classes 12
Class VariablesClass Variablesclass Song @@plays = 0 # Play count for ALL songs def initialize(name, artist, duration) @name = name @artist = artist @duration = duration @plays = 0 # Play count for THIS song end def play @plays += 1 @@plays += 1 "This song: #@plays plays. Total #@@plays plays." endend
Ruby Classes 13
Class MethodsClass Methods Some methods need to be run without being
attached to any particular instance Example: Test if a file is readable
• Can’t open the file if it is not readable• File.readable?(“Filename”) might be defined to
allow this test without any particular file variable
Defined as Classname.methodNameclass Example def instMeth # instance method end def Example.classMeth # class method endend
Ruby Classes 14
Class ConstantsClass Constants Recall that constants begin with uppercase
letters Constants defined outside of any class are
global, while those defined within a class are local:class SongList MaxTime = 5*60 # 5 minutes
def SongList.isTooLong(aSong) return aSong.duration > MaxTime end end
Ruby Classes 15
Variables in RubyVariables in Ruby Variables in Ruby hold references to objects!
• A reference is basically an address with some class/type information
This can make assignment somewhat tricky!
See variables.rb
Ruby Classes 16
SingletonsSingletons Suppose you want only one object of a
particular class, and every time a new instance is “created” it refers to the same object?• This is a design pattern called a singletonclass Logger private_class_method :new @@logger = nil def Logger.create @@logger = new unless @@logger @@logger endend
Logger.create.id » 537762894 Logger.create.id » 537762894
Makes the new() method private
Use Logger.create() instead.
Ruby Classes 17
Alternative constructorsAlternative constructors New calls initialize() with the same parameters
passed to itself. You can call new from other methods:
class Shape def initialize(numSides, perimeter) # ... endendclass Shape def Shape.triangle(sideLength) Shape.new(3, sideLength*3) end def Shape.square(sideLength) Shape.new(4, sideLength*4) endend
Ruby Classes 18
Access ControlAccess Control Access control to methods is C++-likeclass MyClass def method1 # default is 'public' #... end
protected # subsequent methods will be 'protected' def method2 # will be 'protected' #... end
private # subsequent methods will be 'private' def method3 # will be 'private' #... end
public # subsequent methods will be 'public' def method4 # and this will be 'public' #... endend
Ruby Classes 19
Alternate notation for access controlAlternate notation for access control Alternately, you can do it this way:
class MyClass
def method1 end
# ... and so on
public :method1, :method4 protected :method2 private :method3end
This is the notation for a Ruby symbol. We’ll discuss symbols in more detail later…
Ruby Classes 20
Method AccessMethod Access Public – available to anyone Private – only available to other methods of this
class called on the same object Protected – available only to other methods of
this class for this and other objectsclass Account attr_reader :balance
protected :balance
def greaterBalanceThan(other) return @balance > other.balance endend
balance() is available to any Account object
Ruby Classes 21
ExercisesExercises Create a Ruby class for Students
• Name, SSN, midterm, final, array of lab scores, array of hw scores
• Constructor, attr_readers and attr_writers• Virtual attributes exam_avg, lab_avg (read-only)• Class variable: student_count
Read 5 or 10 student records from a file into an array of student objects, then print them